This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A crucial gap for effective in vivo imaging technologies in stem cell research is the need to improve detection sensitivity and to ensure that images can accurately quantify the number of cells at any given anatomical location. Nuclear medicine techniques, especially positron emission tomography (PET) have much higher sensitivity than MRI, and unlike optical imaging, which has similar sensitivity, can provide 3D quantitative images. Outcomes using PET imaging can also be translated from animal models to humans. The overall goal is to develop sensitive in vivo imaging methods to monitor and track transplanted human stem and progenitor cells and overcome current barriers that advance the use of these unique cell populations for regenerative medicine purposes.